Apparatus for inertially deriving ground track angle



Nov. 7,1967

J. N. wl-:IKERT APPARATUS FOR INERTIALLY DERIVING GROUND TRACK ANGLE Filed Jan. 27, 1966 INVENTOR.

JAMES N. WE IKERT ATTORNEY United States Patent Oiiice 3,351,276 Patented Nov. 7, 1967 3,351,276 APPARATUS FR INERTIALLY DERIVING GROUND TRACK ANGLE James N. Weikert, Boston, Mass., assignor to the United States of America as represented by the Secretary of the Navy Filed Jan. 27, 1966, Ser. No. 523,465 Claims. (Cl. 23S-61) is necessary not only to know the longitude and latitude of the aircraft, but it is also ynecessary to know the heading or direction from true North along which the aircraft is flying and the speed of the aircraft. The heading information, however, is not always a true indication of the aircrafts flight path valong a ground reference since high winds or gales tend to push or slide the aircraft off course.

This presents a serious problem to the navigator who must maintain an accurate source along a ground reference since as the aircraft is proceeding along its normal heading, winds are constantly sliding the aircraft off in a different direction. In order to correct for this error, it is necessary for the navigator to constantly check and recheck his position in latitude and longitude to assure a proper flight path along the ground reference.

In the case of an aircraft which is carrying bombs to a target, it is particularly important that the aircraft maintain a true heading with respect to the target location so that the bombs will be dropped on the target. Similarly, in the case of photo reconnaissanceflights it is necessary to maintain a straight flight terns over the .target area will be obtained and consequently a true picture of the photographed area will result.

At present the navigator of the aircraft is required to make numerous computations of his position so as to determine the correct heading of the aircraft. Such computations are not only tedious and subject to error, but also require considerable time to determine the actual heading of the aircraft. The prior art has attempted to solve the aforementioned problem by using inertial systems which resolve the X and Y velocity components of an inertial platform by the wander angle (beta) which is the deviation of the X axis fromy thetrue. North to determine the true North and kEast components of theaircraft velocity. This information, however, still remains in vector coordinate form and its use by a bomber or navigator requires calculations to determine the actual ground track angle and ground speed.

The present invention solves the aforementioned problems by providing automatic means for computing both the ground track angle and the aircraft ground speed. The present invention also provides` for a digital readout display of the ground track angle and visual indication of the aircraft ground speed. v

An object of the present invention is therefore to pro-V vide automatic means for computing the ground track angle and to provide a visual display to the navigator or bomber of the true ground track angle.

Another object of the present invention is to provide a continuous indication of the aircraft ground speed along its particular heading. Additionally, it is to provide an easily adaptable device which may be connected into present inertial systems for indicating aircraft ground track angle and ground speed information.

pattern so that parallel grid pat- Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIG. 1 shows a schematic and block diagram of one embodiment of the invention; and

FIG. 2 illustrates a vectorial representation of the aircraft ground speed and ground track angle.

The present invention comprehends, among other things, the means for inertially determining ythe ground track angle and ground speed of an aircraft.

Referring now to the drawing, there is shown in FIG. 1, an inertial platform and associated sensory equipment 11 which are located within an aircraft. The function of the inertial platform is to provide an indication of the X and Y orthogonal components of velocity of the aircraft with respect to true North and true East; however, due to inherent errors in the gyroscopic equipment located within -the platform, the X and Y components of velocity de-V rived from the platform deviate from the true North and true East by a small amount. The Vdeviation from true North is commonly called the wander angle and could represent a considerable error in the Hight path of the aircraft, and accordingly, cause the aircraft to Veer off course. In order to eliminate this error, a resolver 13,

with the detailed structure shown in resolver 25,v is employed in such va manner as to rotate the X and Y velocity component vectors from the inertial platform into the true North and East components of velocity. This rotation is accomplished iu the following manner.

The X and Y orthogonal velocity components from the inertial platform 11 are coupled toa pair of identical amplifiers 12 through resistors 14 and 15. The twin amplifiers 12 provide both voltage and power gain to the signals from the linertial platform 11 to drive the stator windings of the resolver 13.

The basic operation of resolver 13 is exemplified by its ability to shift the phase angle of the orthogonal signals appearing on the stator windings by an amount equal to the angle of rotation of the rotor.

For example, referring more particularly to the detailed structure shown in resolvervZS, if a pairof orthogonal input voltages is applied to stator windings 27 and 28, the output of rotor windings 36 and 37 will also be a pair of orthogonal signals, but shifted in phase vwith respect to the input signals by an amount equal tothe angular displacement of the rotor 30.

Resolvers 13 and 25 are typically of the compensated type; that is, compensator windings 32 andk 33 are employed in a negative feedback amplifier circuit to stabilize the resolver operation against temperature variations and changes in the resolverfparameters.

Twin amplifier 12 is connected to the stator windings of lresolver 13 through conductors 1'7 yandi18 and the compensation signals from resolver 13'are connected to twinrk amplifier 12 through conductors 19 and 2.1 so as to provide negative feedback to the amplifiers an-d thereby improve the accuracy of resolution of resolver 13.

The X .and Y velocity components shown as Vpx and -Vpy, respectively,

as it is also referred to, may utilize a compass 20 to prothe angular deviation of the X aXis of the inertialy vide a rough physical Ialignment of the platform prior to mathematical gyrocompassing. The compass is merely illustrative of one technique which may be used for this caging operation, others being optical alignment and physical alignment, as descri-bed in the above text.

The output from the sensing equipment is then used to rotate the rotor of resolver 13 in such -a manner as to cause the X and Y velocity components Vpx and Vm., respectively, to be rotated by an amount equal to the wander angle The outputs of resolver 13 are then the true North and East components of velocity relative to the earths reference, as shown in FIG. 2 Iby vectors VEN and VEE, respectively. A better understanding of how the X and Y velocity components of the inerti-al platform are resolved to the true North and East components of the aircraft velocity can be had by an understanding of the analytical relationship between these vectors. The following equations are used to describe the aforementioned relationship:

(a) VEN=Vpx cos -Vp, sin (b) VEE: Vpy cos -l-Vpx sin From the foregoing equations it can be seen how X and Y velocity components from the inertial platform 11 are resolved into the true North and true East velocity components VEN and VEE, respectively. These outputs are coupled to a twin amplifier 24 through conductors 22 and 23. Twin amplifier 24 is substantial-ly similar to twin amplifier 1'2 and provides sufficient amplification to control a resolver 25. Stator windings 27 and 2S are connected to the output of amplifier 24 through conductors 29 and 31. Compensat-ing windings 32 and 33 are connected to amplifier 24 through conductors 34 and 35, Rotor windings 36 and 37 are wound on rotor 30 and are connected to the outputs of resolver 25.

Resolver functions in a similar manner to resolver 13 and accepts the inputs VEN and VEE and resolves them into the resultant vector VG which is a measure of the aircraft ground velocity. The ,angular displacement of vector VG from the true North direction is shown as rpG and represents the true ground track angle of the aircraft.

A better understanding of how resolver 25 resolves the VEN and VEE components into the resultant vector VG and the associated angle rpG can best be expressed by the following equation:

As shown in Equation (c) the resultant vector VG and its associated angle rpG is equivalent to the vectorial summation of VEN and VEE. i

The outputs of resolver 25 are coupled to the inputs of amplifier 41 through resistors 38 and 39. One input of amplifier 41 is grounded so as to provide a reference level for the voltage appearing at an indicator 42 which provides an indication of the aircraft ground velocity VG.

Amplifier 4l is used to drive a field winding 43 of a motor 45. Field winding 43 has a center tap which is connected to a source of D.C. voltage to provide the D.C. field current. A source of alternating current voltage is used to energize an excitation winding 44 of motor 45. A rotor 46 of motor 45 is mechanically connected to a digital readout indicator 47 vwhich provides an indication of the ground track angle rpG. Rotor 46 is also mechanically connected to rotor of resolver 25 and causes the resolver rotor windings to rotate in accordance with the rot-ation of the rotor 46, thereby causing the voltage appearing on rotor winding 36 to be nulled. The voltage null is achieved as a result of the closed feedback loop around resolver 25, amplifier 41 and motor 45. The digital readout indicator 47 will then indicate the ground track angle of the system and the meter 42 will indicate the ground velocity.

The operation of the invention will now be described with particular reference to its application in determining the ground track angle and ground speed of an aircraft.

Assume that a navigator is attempting to maintain an aircraft on la straight `course over a particular terrain and at the altitude at which the aircraft is flying, there are strong winds or gales from such a direction as to tend to slide the aircraft off course. Assume further that the aircraft is flying along a course as shown in FIG. 2 which is generally indicated as the aircraft axis and that the wind direction forms an acute angle with said axis as measured with respect to true North. Assume further that the inertial platform 11 is oriented as shown in FIG. 2 and that the X and Y velocity components from the inertial platform are not properly oriented with respect to true North and true East and that a wander angle does in fact exist. The present invention will then resolve these components into the aircraft ground speed and ground track angle as indicated below.

The X and Y velocity components from the inertial platform are coupled through amplifier 12 to the resolver 13 which resolves these components into the true North and East components VEN and VEE, respectively. These components are then coupled through amplifier 24 to resolver 25 where they are resolved into the true ground speed and ground track angle. The ground speed is recorded on indicator 42 and the ground track angle is recorded on digital readout indicator 47. In this manner the navigator is provided with a continuous visual display of the aircraft ground speed and ground track angle. If the aircraft is moved off course as a result of the high gales or wind-s, he can immediately make the necessary correction to his navigation equipment so as to maintain the aircraft on a proper course, without making any computations.

The present invention accordingly provides automatic means for computing both the ground track angle and the aircraft ground speed so that i-t is readily usable by the navigator by mere visual inspection.

For the purpose of further explaining the operation of the invention, the following example will be used to illus-trate how the aircraft ground track angle and velocity are determined. Assume that the X axis velocity cornponent from the inertial platform Vpx is 183 miles per hour, and that the Y component Vpy is 407 miles per hour. These components may then be resolved into the true North and East components of velocity by resolver 13. For example, if the wander angle is 10 degrees, the true East and true North components of velocity may be determined by using Equations (a) and (b) and substituting the aforementioned numerical Values for Vpx, Vpy and It will then be found that VEN is equivalent to 109.5 miles per hour and VEE is equivalent to 433 miles per hour. By substituting the values thus obtained for VEN and VEE into Equation (c), the resultant vector VG and the associated ground track angle rpG will be determined. In this case it will be found lthat VG is equivalent to 448 miles per hour and rpG is equivalent to 75.9 degrees East -of North. As the aircraft either increases or decreases its speed or deviates from a prescribed course, these changes will be reflected in the digital readout of the ground track angle and the visual display of the aircraft ground speed.

From the above description it can be readily appreciated that the present invention describes a novel means for determining ground track angle and ground speed of an aircraft and displaying this information to a navigator so that the aircraft can be navigated over a prescribed cour-se with considerable accuracy.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A device for deter-mining the ground track angle of an aircraft comprising:

an inertial guidance system having orthogonal velocity component 4output-s with respect to true North in a coordinate frame of reference;

means for computing the angular deviation of said velocity components from true North; and

resolver means receiving said velocity component outputs and responsive to said angular deviation for providing said ground track angle.

2. The device of claim 1 wherein said resolver means comprises:

a first resolver receiving said velocity component outputs `from ysaid inertial guidance system for providing the true orthogonal components of aircraft velocity; and

a second resolver receiving the true orthogonal cornponents from said resolver for providing said ground track angle.

3. The device of claim 2 further comprising:

an indicator means coupled to said second resolver for displaying said ground track angle.

4. The device of claim 2 wherein said means for computing comprises:

means responsive to said inertial guidance system for providing a continuous computation of the deviation from true North of the coordinate frame of reference to said rst resolver. 5. The device of claim 4 further comprising: means connected to the output of said second resolver 5 for displaying said ground speed.

References Cited UNITED STATES PATENTS Re. 25,097 10 2,870,979 2,908,902 2,914,763 3,002,282 3,029,016 l5 3,087,333 3,147,626 3,158,340 3,205,346 3,217,150

Ciscel.

Tribken et al.

Gray et al 33-222.75 Greenwood et al 73-178 ,Rumrill 235-61 Shapiro et al. n

Newell 235-61 Fischer et al 33-226 Sellers.

Wright et al 23S- 61 Wright et al 23S-61 2 RICHARD B. WILKINSON, Primary Examiner. S. A. WAL, H. B. KATZ, Assistant Examiners. 

1. A DEVICE FOR DETERMINING THE GROUND TRACK ANGLE OF AN AIRCRAFT COMPRISING: AN INERTIAL GUIDANCE SYSTEM HAVING ORTHOGONAL VELOCITY COMPONENT OUTPUTS WITH RESPECT TO TRUE NORTH IN A COORDINATE FRAME OF REFERENCE; MEANS FOR COMPUTING THE ANGULAR DEVIATION OF SAID VELOCITY COMPONENTS FROM TRUE NORTH; AND 